1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer or the like, and, more particularly, to an image forming apparatus in which a corona charger is used to charge an image bearing member.
2. Description of Related Art
FIG. 1 is a cross-sectional view showing, by way of example, the essential parts of an electrophotographic-type image forming apparatus.
Referring to FIG. 1, a photosensitive drum 1, which serves as an image bearing member, has a photoconductive layer provided on a cylindrical conductive base member thereof, and is so supported as to be rotatable in the direction of an arrow R1.
Around the photosensitive drum 1, there are disposed, in order along the rotating direction thereof, a scorotron charger 2 for charging the surface of the photosensitive drum 1, an exposure device for reading an original to produce an image signal and exposing the photosensitive drum 1 on the basis of the image signal to form an electrostatic latent image on the photosensitive drum 1, a developing device 4 for causing a toner to adhere to the electrostatic latent image to form a toner image on the photosensitive drum 1, a surface-potential sensor 5 for detecting the surface potential of the photosensitive drum 1 in the vicinity of the developing device 4, a transfer charger 8, such as a corona transfer charger, for transferring the toner image formed on the photosensitive drum 1 to a transfer sheet P, which serves as a transfer material, a separation charger 9, such as an electrostatic separation charger, for separating the transfer sheet P having the toner image transferred thereto from the photosensitive drum 1, a cleaning device 13 for removing toner particles remaining on the photosensitive drum 1 even after the transfer of the toner image, an exposure lamp 30, which serves as a pre-exposure device, for removing charges remaining on the photosensitive drum 1, and so on.
The transfer sheet P having the toner image transferred thereto is transported to a fixing device 12 after having been separated from the photosensitive drum 1. At the fixing device 12, the toner image on the surface of the transfer sheet P is fixed to form a desired print image. Then, the transfer sheet P is discharged to the outside of the main body of the image forming apparatus.
Further, an image scanner unit 18 is provided for reading an original 15 placed on an original-placing glass board 14 by scanning the original 15 with an illuminating lamp 16, and for converting image information obtained by the scanning into an electrical signal by using a photoelectric conversion element 19. In the image scanner unit 18, reflected light from the original 15 scanned with the illuminating lamp 16 is guided to a lens 19 by mirrors 17a, 17b and 17c and is then imaged on the photoelectric conversion element 19 by the lens 17d.
The electrical signal obtained by the photoelectric conversion element 19 is digitized by an A/D converter 21, and is then converted by an image signal processing part 22 into an image signal represented with 256 gradation levels of from "0" (00hex) to "255" (FFhex) proportional to individual image density levels.
The image signal outputted from the image signal processing part 22 is supplied to a laser driver 24. The laser driver 24, which serves as a signal generating part, modulates the light emission of a laser 20 in accordance with the image signal.
A laser beam 3 obtained by the light emission modulated in accordance with the image signal is made incident on the photosensitive drum 1 via a polygonal mirror 28 and a mirror 17e, so that an electrostatic latent image is written to the photosensitive drum 1.
The photosensitive drum 1 may have good charging capability or poor charging capability due to uneven products thereof.
Further, since the charging capability of the photosensitive drum 1 depends on the amount of light of the pre-exposure, the charging capability of the photosensitive drum 1 varies according to the unevenness of the amount of light of the pre-exposure or the decrease of the amount of light of the pre-exposure due to the repeated usage.
In addition, the charging capability of the photosensitive drum 1 varies according to the variation of the discharging characteristic of the scorotron charger 2, the variation of the charging characteristic of the photosensitive drum 1, etc., due to the repeated usage or the variation of an environment where the image forming apparatus is used.
In order to absorb the above-mentioned unevenness or variation, such a technical art is known that the surface-potential sensor for detecting the surface potential of the photosensitive drum 1 is provided in the image forming apparatus and is used to vary a voltage to be applied a grid wire 2a of the scorotron charger 2 in such a way as to keep the surface potential of the photosensitive drum 1 at a desired potential. This technical art is based on the fact that the surface potential of the photosensitive drum 1 depends on the potential of the grid wire 2a.
However, the scorotron charger 2 has to be made detachably mountable with respect to a power-feeding contact portion of the main body of the image forming apparatus, in consideration of service maintenance or the like of the main body of the image forming apparatus. Accordingly, if the scorotron charger 2 fails to be securely set to the power-feeding contact portion of the main body of the image forming apparatus, there may occur such a state that only a contact of the current receiving portion for the charging wire of the scorotron charger 2 is connected to the power-feeding contact portion of the main body of the image forming apparatus, while a contact for the potential of the grid wire 2a does not come into contact with the power-feeding contact portion of the main body of the image forming apparatus.
In the above state, the current in the charging wire would flow directly to the photosensitive drum 1, thereby causing leakage on the surface of the photosensitive drum 1, so that it may be expected that the photosensitive layer of the photosensitive drum 2 is damaged.